CBP/p300 are important coactivators for several transcription factors including CREB, nuclear hormone receptors, NF- B and p53. Recent experiments suggest that CBP/p300 mediate gene activation through effects on chromatin remodeling, but interactions with basal transcription factors have also been proposed. To analyze CBP transcription activation, we have utilized an in vitro transcription assay that allows the analysis of transcription initiation and reinitiation in the absence of chromatin effects. Addition of Tax and a N-terminal fragment of CBP (amino acids 1-682) to the in vitro transcription reactions increased both full-length and shorter transcripts resulting from initiation and reinitiation. A CBP deletion mutant lacking the N-terminal activation domain was inactive. Analysis of the interaction of Tax with full-length endogenous CBP indicates that a transcriptionally active complex of Tax/CBP/RNA pol II can be immunoprecipitated from cell extracts. We propose that CBP activates transcription in vivo not only by loosening chromatin structure by its histone acetylase function, but also by directly promoting transcription initiation and reinitiation through Tax-responsive CREB binding.